Organic electroluminescent elements have received attention as next-generation display elements, and the development of various organic materials used in the luminescent elements has been promoted actively in recent years. Particularly, attention has been focused on phosphorescent materials that utilize luminescence from an excited triplet state, as luminescent materials from the viewpoint of improvement in luminous efficiency.
In the case of using luminescence from an excited singlet state, the limit of external quantum efficiency is allegedly 5% because the generation ratio between a singlet exciton and a triplet exciton is 1:3 and thus the generation probability of luminescent excited species is 25% and because light extraction efficiency is approximately 20%. On the other hand, if even an excited triplet state can be utilized in this, luminous efficiency becomes four times in principle compared with the case of the excited singlet because the upper limit of internal quantum efficiency becomes 100%; thus the development of phosphorescent materials has been performed actively. Although iridium complexes having a 2-phenylpyridine-based ligand have received attention so far as typical phosphorescent materials, the development of novel phosphorescent materials has been demanded with the future objective of further improvement in luminous efficiency and durability.
Iridium complexes having a 2-phenylpyrimidine-based ligand similar to a compound of the present invention are described in Patent Literatures 1 to 7 and Non Patent Literatures 1 to 3. However, the problems to be solved, such as luminous efficiency or durability, still remain, and more thermally stable materials exhibiting high luminous efficiency have been demanded. Moreover, according to the findings of the present inventor, there were problems about the iridium complexes having a 2-phenylpyrimidine-based ligand compared with 2-phenylpyridine-based iridium complexes: solubility in a solvent was low and workability and operability were significantly poor.
Although an iridium complex in which a trifluoromethyl group is introduced in position 5 of the pyrimidine ring (see a diagram below) of a 2-phenylpyrimidine ligand (formula (A)) is disclosed in Non Patent Literature 1, its thermal stability is low (see [0171]) and in addition, its luminescence is limited to a green color (λmax=525 nm).

An iridium complex in which a phenyl group or a thienyl group is introduced in position 5 of the pyrimidine ring of a 2-phenylpyrimidine ligand (e.g., formula (B)) is disclosed in Non Patent Literature 2. However, the emission wavelengths of these iridium complexes are 522 to 558 nm and their colors of luminescence are limited to green to orange regions. Particularly, for the iridium complex of the skeleton described in Non Patent Literature 2, it is difficult to emit light in a blue region. Moreover, according to Non Patent Literature 2, there are problems: the emission quantum yields of these iridium complexes in toluene are 0.052 to 0.34 and are still low.

An iridium complex in which a methyl group is introduced in position 4 of the pyrimidine ring of a 2-phenylpyrimidine ligand (formula (C)) is disclosed in Patent Literature 7. However, using the ligand of this type, there is the possibility that isomers are generated depending on coordination patterns to iridium (see a diagram below), and there are problems: it is difficult to synthesize the intended iridium complex with high purity. Also, there is the possibility that contamination with various isomers have adverse effect on luminescent elements.

Moreover, phosphorescent materials exhibiting a high emission quantum yield in a solid state are preferable for solid devices such as organic electroluminescent elements, and further, phosphorescent materials highly soluble in a solvent have been desired strongly in the case of producing films of these phosphorescent materials by a coating method. As described above, the 2-phenylpyrimidine-based iridium complexes described in Patent Literatures 1 to 7 and Non Patent Literatures 1 to 3 still have room for improvement from the viewpoints described above. Although search for novel phosphorescent materials has been performed in such technical background, there is no description about the luminescence properties of an iridium complex in which an alkyl group having 2 to 30 carbon atoms which may have a substituent is introduced in position 5 of the pyrimidine ring of a 2-phenylpyrimidine ligand, and the properties of a luminescent element using it.